(i) Field of the Invention
This invention relates to electrodes employed in electrolytic cells of the type used for manufacture of products, e.g., chlorates, perchlorates, persulphates and hydroxides. It relates more specifically to cathodes either alone or as a bipolar electrode including a cathode section and an anode section. The electrode of this invention is utilizable for the production of those products which involve generating hydrogen electrochemically on the surface of the cathode.
(II) Description of the Prior Art
The cathodes used in early prior art for electrochemical technology were, in most cases, of iron or steel. Later on, graphite was used in many electrolytic cell designs employing bipolar electrodes. In more recent years, since 1965, the cathodes employed commercially have mainly been of steel, combined with anodes fabricated from titanium metal which have been specially surface coated. One multi-electrolytic cell, employing such new anodes combined with steel cathodes, is shown in Canadian Pat. No. 914,610 issued to G. O. Westerland. Another electrolyzer which preferably employs cathodes of steel or other ferrous metal is described in U.S. Pat. No. 3,948,748 issued to Messner et al. Still another, a monopolar type cell, in U.S. Pat. No. 3,598,715 by D. N. Goens, describes a design with cathode assemblies of expanded mild steel. Other cathode materials used in recent technology include the mercury cathode in the electrolytic process of preparing pure hydrogen, in U.S. Pat. No. 3,458,412 issued to Matsuaki Shinagawa et al.
Copending U.S. application Ser. No. 618,078 filed Sept. 30, 1975, now U.S. Pat. No. 3,994,798 provided a teaching of several cathodic materials. Thus, quite generally, that application taught the use of a "suitable cathodic material," which was defined as a material which was electrically conductive, or substantially insoluble in the electrolyte under cathodic conditions, was resistant to reduction, and either was substantially impermeable with respect to H.sub.2, or if permeable by H.sub.2, was dimensionally stable with respect to H.sub.2. Steel was taught to be the preferred material, but it would also be possible to use copper, chromium, cobalt, nickel, lead, tin, iron or alloys of the above metals.
It was, however, also taught that an anode/cathode could be provided from titanium or a titanium alloy. In addition, other metals taught for use as such electrode included tantalum, zirconium and columbium and alloys of such metals. It was taught that, in performing as a cathode, the titanium formed a hydride and consequently some corrosion could occur should the electrolyte temperature be excessive (i.e., above about 100.degree. C.) and equalization of electrical potential in the cell under such circumstances would be poor.
Another anode/cathode taught in that application was one where the anodes employed were of titanium, which was surface coated with platinum to improve anode performance. The cathodes employed were of titanium, which was surface coated or treated to improve their cathode performance as cathode surface by the use of a coating of a "suitable cathodic material" (as heretofore defined). For example, titanium sheet of about 1.5 mm thick having a low carbon steel cathode surface was welded and successfully used as the cathode. The coated electrodes could be made using the explosion bonding technique described in Canadian Pat. No. 760,427 issued June 6, 1967 to Ono et al.
That application also taught the use of a titanium cathode comprising a grit blasted solid sheet, e.g., the use of a grit of aluminum oxide to increase the surface area and a powdered metal porous sheet. However, prolonged use of these cathodes has shown a tendency or erosion and warpage with resulting risk of electrical short circuiting to the adjacent anode.
An improvement thereon was provided in Canadian patent application Ser. No. 232,349 filed July 28, 1975. It was taught that the titanium cathodes could be of the solid sheet, grit blasted type or of the porous or semi-porous fused powdered metal sheet type. Some advantages pointed out for these cathodes were no substantial corrosion; no significant amount of impurities from the cathodes into the electrolyte product; provide for a welded joint for minimum electrical resistance; no cathodic protection hardware required to protect cathodes against corrosion during shut downs; and dimensional stability.
It has been found that while these advantages are true under ideal conditions, nevertheless the cathode is prone to deteriorate drastically in a relatively short period of time, approximately the year's operation, if the conditions are somewhat harsh (i.e., high current density, small spacing, high temperature, high current concentration).
It is also now known that a balancing must be made with respect to pore size. A smaller pore size is desirable for better cathode performance in regard to ohmic resistance. The surface voltage actually is better for the larger pore size. Thus, the cathode should have a small pore size for better conductance of current but a large pore size for improved surface voltage.
It has also been found that, while the cathodes are dimensionally stable if the current distribution is even, in practical application, especially towards the end of an anode life cycle, the current distribution is not uniform over the cathode sheet. This results in warpage which in some cases may be very significant and require a special designed electrode assembly design to prevent electrical short circuiting. Furthermore, in order to re-use the cathode, it may be necessary to heat and press the sheet flat. The powdered titanium cathode, pressed and fused to a porous or semi-porous sheet, is less inclined to warpage since it is likely to be several times the thickness (i.e., in cases where current flows longitudinally through the sheet and voltage drop is maintained the same) compared to the grit blasted titanium sheet cathode.
Moreover, even with solid sheet titanium cathodes there is a tendency to lose thickness after some months' operation due to erosion of the film formed onto the surface of the sheet. This may be very significant after a year of operation.